1. Field of the Invention
The present invention relates to a power supply device, more particularly to a power supply device that employs resonant converting circuits.
2. Description of the Related Art
Resonant converters are known to have advantages such as high conversion efficiency and low cost, and are hence commonly used in high-power isolated DC/DC conversion. Nevertheless, because resonant converters operate on sinusoidal current waveforms, output current after rectifier always has high ripple factor. Especially for applications that require high output power, a plurality of parallel-connected capacitors are required to meet requirements of ripple factor and voltage stress. Conventional technique for reducing ripple factor of LLC converters is to realize phase-shifting output currents of parallel-connected resonant converters. However, resonant converters are controlled through frequency modulation and cannot be controlled through control technique of pulse-width-modulation (PWM) type DC/DC converters. Therefore, control of parallel-connected resonant converters to achieve low ripple and uniform current output has been a difficulty.
Referring to FIG. 1, a conventional power supply device 900 includes a first resonant circuit 91 and a second resonant circuit 92 that are coupled in parallel and that provide an output voltage VO. The conventional power supply device 900 has a current control loop and a voltage control loop.
The current control loop is for determining an optimal operating frequency at which the first and second resonant circuits 91, 92 operate. The current control loop includes a subtractor 96 and a load-balancing controller 97. The subtractor 96 generates a signal corresponding to a difference between output currents IOA, IOB of the first and second resonant circuits 91, 92. The load-balancing controller 97 generates a driving signal, according to the signal generated by the subtractor 96, for controlling switching frequency of power switches (not shown) in the first and second resonant circuits 91, 92, thereby controlling the output currents IOA, IOB.
The voltage control loop is for stabilizing the output voltage VO of the first and second resonant circuits 91, 92. The voltage control loop includes a voltage controller 93 that samples the output voltage VO, that generates a control signal (D) corresponding to the output voltage VO, and that provides the control signal (D) to a buck converter 94 so as to control switching frequency of a power switch unit (not shown) therein. The power switch unit in the buck converter 94 is controlled in a manner that the buck converter 94 converts the output voltage from a power factor corrector 95 into an input voltage, which is provided to the first and second resonant circuits 91, 92, such that the first and second resonant circuits 91, 92 provide a required output voltage VO.
Nonetheless, the conventional control method requires sampling of the output currents IOA, IOB of the first and second resonant circuits 91, 92, and is therefore difficult to implement. Furthermore, the buck converter 94 reduces power conversion efficiency of the power supply device 900. Moreover, the conventional control method requires that an optimal operating frequency (frequency at which gains of the first and second resonant circuits 91, 92 are substantially identical) be determined such that the output currents IOA, IOB are balanced, which can be difficult to achieve and is unfavorable in optimizing efficiency of the first and second resonant circuits 91, 92.